1. Field of the Invention
This invention relates to a tuner, and more particularly, to a combination tuner which is used to select the band of a desired channel from a plurality of UHF and VHF bands and a suitable circuit element for receiving television signals of the selected channel.
2. Description of the Prior Art
A reduction in size of television receivers is going on rapidly, as is evident from the appearance of a television receiver employing a liquid crystal display. This has created a strong demand for smaller television tuners. The most effective and economical way to reduce the size of a television tuner is to decrease the number of its parts. Therefore, it is usual to employ a common transistor in a high-frequency amplification circuit and a common varactor diode in a local oscillation circuit for both UHF and VHF bands, as shown by way of example in FIG. 2. The device of FIG. 2 also includes a bias circuit not shown.
In FIG. 2, TR is an amplifying transistor having a grounded base, C.sub.1 to C.sub.3 are capacitors, VRD is a varactor diode grounded at one end, DCL is a distribution constant line serving as a resonant line for the UHF bands, L.sub.1 is a resonant coil for high VHF channels (channels 4 to 12), L.sub.2 is a resonant coil for low VHF channels (channels 1 to 3), and SW.sub.1 and SW.sub.2 are diodes for switching the resonant band. The distribution constant line DCL and the resonant coils L.sub.1 and L.sub.2 are connected in series to one another. The distribution constant line DCL has one end connected to the collector of the transistor TR and the cathode of the varactor diode VRD through the capacitors C.sub.2 and C.sub.3, respectively. The junction between the distribution constant line DCL and the resonant coil L.sub.1 is grounded through the switching diode SW.sub.1. The junction between the resonant coils L.sub.1 and L.sub.2 is grounded through the switching diode SW.sub.2. The opposite end of the coil L.sub.2 from its junction with L.sub.1 is also grounded.
The varactor diode VRD has a limited capacity change which is not sufficient to cover the whole range of UHF and VHF bands. Therefore, the whole range is divided into three bands, i.e., (1) UHF channels, (2) low VHF channels (channels 1 to 3), and (3) high VHF channels (channels 4 to 12), and a tuning circuit which is suitable for a desired band is formed to enable the use of the transistor TR and the varactor diode VRD in common for the UHF and VHF bands. More specifically, if the switch SW.sub.1 is turned on, the varactor diode VRD and the distribution constant line DCL form a resonant circuit for tuning an UHF band, or if the switch SW.sub.2 is turned on, while the switch SW.sub.1 is turned off, the varactor diode VRD and the resonant coil L.sub.1 form a resonant circuit for tuning a high VHF channel, or if both of the switches SW.sub.1 and SW.sub.2 are turned off, the varactor diode VRD and the resonant coils L.sub.1 and L.sub.2 form a resonant circuit for tuning a low VHF channel. A voltage corresponding to a desired channel is applied to the cathode terminal of the varactor diode VRD.
The system as hereinabove described has the advantage of enabling the selection of the three resonant circuits by the two switches. When an UHF band is selected, however, the resonant circuit, which is shown at (A) in FIG. 3, gives so large a loss that its Q-factor under no load is lowered and its characteristics are impaired to a practically unacceptable extent. This is due to the voltage and current distributions of the distribution constant line DCL which are shown at (B) and (C), respectively, in FIG. 3. The current becomes the highest at the switch SW.sub.1 which is a switching diode having a high series resistance, and the resonant circuit shows the largest loss at the switch SW.sub.1.